1. Field of the Invention
The present invention relates to a method and apparatus for supplying gaseous raw materials which are used to manufacture fused silica glass preforms for optical fibers and also to manufacture semiconductor devices.
2. Description of the Related Art
Heretofore, various gaseous raw materials have been used in manufacture of fused silica glass preforms for optical fibers and also in manufacture of semiconductor devices. It is essential to accurately control the flow rates of supplied gases. Generally, in order to place emphasis on the accuracy of the flow rates of the supplied gases, one installation is equipped with one set of raw material supply equipment to completely prevent the gases against external disturbances from other apparatus. Of course, in some cases, a gaseous raw material is injected into plural evaporators separately. In other cases, a gaseous raw material is automatically introduced from the outside by the use of collecting piping.
Where a gaseous raw material generated by the same evaporator should be used by other users, even if each of them has an accurate flowmeter such as a mass flow controller, a problem takes place. In particular, when one user blocks off or permits supply of the gas into this user, the flow rates of the gas supplied to the other users are affected. As a result, a noise is introduced in the accurate reaction process. This adversely affects the characteristics of the final products. Accordingly, in a reaction system designed to provide high accuracy, the flow rates of the gaseous raw material supplied to plural users are not controlled independently in a time-domain in stages following the evaporator. That is, because the accuracy has problems, even if every user has a mass flow controller, for example, it is customary to make the operations manual so that every manufacturing batch may be placed under the same conditions. During a reaction, the gaseous raw material is sent to users from the same evaporator normally under the same heater conditions and at the same pressure. Where it is necessary for a user to greatly adjust the flow rate of the gaseous raw material or to block off or permit the supply, it is common practice to adjust the heat source of the evaporator. For instance, the electric power fed to a heater is adjusted or stopped. It is known to supply a gaseous raw material to plural users from a single evaporator. In this case, however, the number of the users is limited to two or three. Moreover, the reaction conditions are maintained substantially constant from the start of the reaction for each user until the reaction is completed. The flow rates of the gas supplied to the users, respectively, are not adjusted or blocked off independently.
In addition to the foregoing problems, as high technology has evolved in recent years, the number and kinds of gas installations have increased. If each unit is provided with a separate gaseous raw material supply device, then the installation cost is increased. Also, it is more difficult to secure installation space. Additionally, it is more difficult to manage the installations. In this way, serious problems have occurred.
In high technologies, especially in optical-fiber manufacturing sections and in semiconductor fabrication sections, used raw-material gases and other gases are corrosive compounds such as chlorine, bromine, and fluorine, poisonous gases typified by chlorine, phosphine, and arsine, or etchant gases such as hydrofluoric acid, hydrochloric acid, or nitric acid. In this way, many gases entail dangers when treated. Therefore, management of these dangerous gases has become comparable to, or even has surpassed, management of accuracy in importance.
In view of these circumstances, we have earnestly investigated means for supplying and distributing hazard gases without deteriorating the accuracy of supply of gaseous raw materials, using apparatus that can be managed easily and can be centralized. As a result, we have found a method of supplying a gaseous raw material in such a way that each individual user can independently take out the gas at a desired flow rate at any desired time, by circulating the gaseous raw material generated from an evaporation means through a closed loop without blocking off the flow of the gas and by forming a plurality of takeout ports in the closed loop. The number of raw-material evaporation means which often induce troubles can be reduced.